A spark plug used for ignition of an internal combustion engine, such as an automobile engine, generally includes: a substantially tubular metallic shell; a substantially tubular insulator disposed in a bore of the metallic shell; a center electrode disposed in an axial bore of the insulator at the front end of the axial bore; a metal terminal disposed in the axial bore at its rear end; and a ground electrode with one end joined to the front end of the metallic shell and the other end facing the center electrode so as to form a spark discharge gap. When high voltage is applied between the center electrode and the metallic shell, spark discharge occurs between the center electrode and the ground electrode, and the spark discharge causes fuel in a combustion chamber to ignite.
In recent years, with the need for increasing the output power and efficiency of internal combustion engines of automobiles etc., there is a demand for developing spark plugs reduced in size in order to, for example, reduce the size of the engines themselves and to allow the engines to be designed freely. To reduce the size of a spark plug, it is inevitable to reduce the size of the insulator. However, as the size of the insulator decreases, its thickness decreases, making it difficult to ensure the withstand voltage of the spark plug. To meet the need for increasing the output power of an engine, the discharge voltage of the spark plug tends to increase. Therefore, it is more and more difficult to ensure the withstand voltage.
An object of the invention described in Japanese Patent Application Laid-Open (kokai) No. 2005-129377 is to ensure an appropriate withstand voltage in a small-diameter spark plug having a mounting thread of M10 or less. To achieve this object, a study was conducted to find which part of a ceramic insulator is apt to be subjected to dielectric breakdown. The means for achieving the object that is described in Japanese Patent Application Laid-Open (kokai) No. 2005-129377 (claim 1) is a “spark plug characterized in that a circumferentially extending portion of the ceramic insulator (20) which has a largest diameter and is located within the metallic shell (10) forms a trunk portion (22), a portion adjacent to the trunk portion (22) and extending toward one end (20a) of the ceramic insulator (20) forms a middle portion (23) having a diameter smaller than that of the trunk portion (22), the trunk portion (22) and the middle portion (23) of the ceramic insulator (20) face the inner surface of the metallic shell (10) through a gap, and (D1−D2)/2 is 1.8 mm or less, wherein D1 is the inner diameter of a portion of the metallic shell (10) facing the trunk portion (22) through the gap, and D2 is the inner diameter of a portion of the metallic shell (10) facing the middle portion (23) through the gap.” However, in recent years, the thickness of ceramic insulators tends to decrease more and more, and therefore there is demand for a spark plug with further improved withstand voltage.